The present invention relates to an improved process for the production of linear, branched or cyclic C.sub.1 -C.sub.15 alkyl (meth)acrylates, by a direct esterification of (meth)acrylic acid with the corresponding alcohol, this reaction being catalysed by sulphuric acid.
The problems which arise regarding this production will now be explained, for convenience, on the basis of the example of the direct esterification of acrylic acid with butanol. These problems and the solution proposed by the invention are the same with regard to the use, on the one hand of methacrylic acid and, on the other hand of alcohols other than butanol.
Thus, the industrial process for the production of butyl acrylate involves a reaction of direct esterification of acrylic acid with butanol, which reaction is catalysed by sulphuric acid. In this process, the function of shifting the reaction equilibrium is provided for, not by the addition of a solvent which azeotropically entrains the water of reaction, but by an excess of the esterifying alcohol (in this case, butanol), which has the property of forming an azeotrope with water.
On conclusion of the reaction step which is carried out discontinuously, virtually all of the sulphuric acid has been converted to butyl hydrogen sulphate BuSO.sub.4 H, according to the following esterification reaction of sulphuric acid with butanol: EQU BuOH+H.sub.2 SO.sub.4 .fwdarw.HBuSO.sub.4 +H.sub.2 O
Consequently, the reaction mixture at the end of the reaction contains butyl acrylate, butanol, acrylic acid, butyl hydrogen sulphate, traces of sulphuric acid, and the stabilizers conventionally used in the reaction.
The esterification reaction is followed by purification steps, which are generally carried out in continuous fashion:
the acidic species contained in the crude reaction mixture are neutralized, by addition thereto of an aqueous solution of base (sodium hydroxide); during this step, the acrylic acid is neutralized to give sodium acrylate, the butyl hydrogen sulphate to give neutral butyl sulphate BuSO.sub.4 Na, and the traces of sulphuric acid to give sodium sulphate Na.sub.2 SO.sub.4, all these salts passing into the aqueous phase; PA1 the separated organic phase, obtained after neutralization, is washed with water in an extraction column in order to remove the traces of impurities, of sodium hydroxide and of salts, and the washed butyl acrylate is then sent to a first distillation column, which allows the light products to be removed at the head, which products are recycled to the esterification reaction. The ester freed of this light fraction is then sent to a second column, from which it emerges at the head, purified by removal of the heavy products; PA1 the butanol present is recovered: PA1 neutral butyl sulphate: the COD theoretically expelled is of the order of 2 kg of O.sub.2 per kg of sulphuric acid involved in the esterification reaction; PA1 sodium acrylate: the theoretical COD, due to the presence of residual acrylic acid at the end of the reaction, is approximately 8 kg of O.sub.2 per tonne of butyl acrylate produced, which is approximately equivalent to a quarter of the COD due to the neutral butyl sulphate; it can be increased by a partial hydrolysis of the butyl acrylate in the course of the purification steps of the said ester. PA1 a hydrolysis of the alkyl hydrogen sulphate may be carried out in the organic reaction medium itself at the end of the esterification reaction, as is described in the article by J. WASILEWSKI, G. KESICKA, H. SMOLIK, E. CHROMIAK, Przemysl Chemiczny, 11-12 60, 1981, 541-543. In this article, the authors describe the hydrolysis of 2-ethylhexyl sulphate in the final reaction medium of synthesis of 2-ethylhexyl acrylate, at 80.degree.-105.degree. C., with 7.5% by weight of water, for 15 to 30 minutes. The main disadvantages are: PA1 it is also possible to envisage extraction of the butyl hydrogen sulphate with water, followed by a new extraction with the esterifying alcohol, as featured in German Patent Application DE-A-4,019,781. This approach to the describe problem is not applicable in the case of the process for the production of alkyl (meth)acrylates by direct esterification, because the excess alcohol (butanol) acts as a dissolving intermediary and greatly limits the extraction yield of the butyl hydrogen sulphate into the water. To be applicable, it would be necessary to employ a solvent, which would complicate the process, especially for the recovery and recycling of this solvent. PA1 (a) addition to the said crude reaction mixture of a base to neutralize the (meth)acrylic acid, the C.sub.1 -C.sub.15 alkyl hydrogen sulphate and the traces of sulphuric acid which are present therein, the resulting salts passing into the aqueous phase of the said mixture, the organic phase and the aqueous phase obtained from this neutralization being separated and the alkyl (meth)acrylate sought being recovered from the said organic phase; PA1 (b) recovery of the alcohol from the said aqueous phase, the used waters, which are freed of the said alcohol, being expelled, PA1 the aqueous phase obtained after the actual esterification reaction; PA1 the aqueous phase resulting from the hydrolysis after neutralization; and PA1 the aqueous phase obtained from the abovementioned extraction column.
in the separated aqueous phase obtained after neutralization, which essentially contains neutral butyl sulphate, butanol, sodium acrylate and traces of sodium sulphate; PA2 in the aqueous phase obtained after the actual esterification reaction; and PA2 in the aqueous phase obtained after washing of the organic phase in the extraction column, PA2 a low efficiency of removal of the 2-ethyl hexyl hydrogen sulphate; PA2 hydrolysis of the 2-ethyl hexyl acrylate ester produced, which results in a loss of product and an increase in the pollution expelled in the form of acrylic acid (or sodium acrylate after neutralization); PA2 the need to treat a sizeable flux, since it is all of the crude reaction mixture from the end of the esterification reaction which is treated; PA2 a risk of damaging the quality and the stability of the 2-ethyl hexyl acrylate, which is a polymerizable ester.
this recovery of the butanol being carried out in a distillation column supplied at the head with the waters to be treated, in which the butanol is entrained at the head and can be recycled to the esterification reaction, while the foot of the column consists of the waste waters, which are freed of butanol and which will be passed to the biological treatment plant.
These waster waters thus expelled contain a large amount of organic pollution, measured by the chemical oxygen demand (COD) due to the presence of:
The pollution due to butyl hydrogen sulphate (or the neutral butyl sulphate) could be avoided by using acid catalysts other than sulphuric acid which have the property of not giving an alcoholysis reaction with the alcohol. For example, it is known that methanesulphonic acid (MSA) and para-toluenesulphonic acid (PTSA) do not react, or react to only a small extent, with alcohols, to form alkyl sulphates. However, these catalysts have the disadvantage of being much more expensive than sulphuric acid, which makes it necessary to perform a recovery of these catalysts at the end of the reaction for the purpose of recycling them. This recovery, which could be envisaged by extraction with water, is not suitable here because of the presence of excess alcohol to remove the water of reaction, this alcohol playing the part of a competitive solvent with respect to the water, and thereby limiting the extraction yield of the catalyst into the water.
Other techniques may be envisaged for reducing the pollution due to the alkyl sulphate, but they all have disadvantages: